The present invention relates to an optical disc apparatus, a focus control method, and a control program capable of high-quality reproduction of an optical disc having at least two data layers.
Optical discs for which a laser beam is used in recording/reproduction have been in wide use as a storage medium for audio data, video data, and other several types of digital data. Representative of optical discs are CD (Compact Disc), DVD (Digital Versatile Disc), BD (Blu-ray Disc), etc. Moreover, representative of data-writable optical discs are CD-R, CD-RW, DVD-R, DVD-RW, BD-RE, etc. Among these discs, there are optical discs having several data layers for larger storage capacity. A representative of such optical disc is DVD-DL (Digital Versatile Disc Dual Layer) having two data layers on one side.
When an optical disc is set in an optical disc apparatus, usually, a laser beam is directed onto the optical disc while the focal point is being shifted from the surface side (a laser side) to the far side of the optical disc. The shift of focal point is perform to acquire a characteristic curve of a FE (Focus Error) signal (a focus-error signal curve, referred to as a FES curve, hereinafter) of the optical disc, for discriminating the type of the disc, obtaining the in-focus point, etc.
FIG. 1 shows a FES curve for a regular single-sided dual-layer DVD having data layers separated from each other with an enough distance. For such a single-sided dual-layer DVD, there are two FES curve portions. One FES curve portion corresponds to a layer L0 that is a first data layer on the surface side. The other FES curve portion corresponds to a layer L1 that is a second data layer on the far side. The FES curve portions are connected to form a FES curve. With such a FES curve shown in FIG. 1, several types of control, such as focus control, can be performed with regular procedures.
Most regular optical discs available on the market have a standard distance between layers L0 and L1, the value of the distance being almost the center of a standard range. Not only that, there are dual-layer DVDs having layers L0 and L1 with a smaller distance therebetween than the regular optical discs. Such dual-layer DVDs having closer layers L0 and L1 could exhibit a FES curve, such as shown in FIG. 4 (which will be explained later), with low FE signal levels between the layers L0 and L1. This is caused by overlapping of reflected beams from adjacent layers. With such a FES curve, normal data reproduction may not be performed due to focus error on the layer L1 that is the second data layer.
In order to solve the problem, U.S. Pat. No. 6,392,965 discloses an invention related to an optical pickup device for achieving accurate focus control of an optical disc with close data layers by having main photoreceptive domains and auxiliary photoreceptive domains for mitigating the effect of reflected beams from the close data layers.
However, the invention disclosed in U.S. Pat. No. 6,392,965 has a problem of a higher cost due to increase in the number of divided photoreceptive domains, or cell numbers.